In copying from 𝓞 into 𝓟, Oliver Cowdery initially wrote the relative pronoun that but almost immediately corrected it to which by crossing out the that and supralinearly inserting the which (there is no change in the level of ink flow). Since either that or which is possible here, the critical text will follow the corrected reading in 𝓟. For further discussion of the competition between which and that in the Book of Mormon text, see under which in volume 3.
Summary: Maintain in Jacob 5:57 the relative pronoun which, the corrected reading in 𝓟 (“save it be those which are most bitter”).